Abrupt Climate Change around 4 ka BP： Role of the Thermohaline Circulation as Indicated by a GCM Experiment

A great deal of palaeoenvironmental and palaeoclimatic evidence suggests that a predominant tem-perature drop and an aridification occurred at ca. 4.0 ka BP. Palaeoclimate studies in China support thisdedution. The collapse of ancient civilizations at ca. 4.0 ka BP in the Nile Valley and Mesopotamia hasbeen attributed to climate-induced aridification. A widespread alternation of the ancient cultures was alsofound in China at ca. 4.0 ka BP in concert with the collapse of the civilizations in the Old World. Palaeo-climatic studies indicate that the abrupt climate change at 4.0 ka BP is one of the realizations of the coldphase in millennial scale climate oscillations, which may be related to the modulation of the ThermohalineCirculation (THC) over the Atlantic Ocean. Therefore, this study conducts a numerical experiment ofa GCM with SST forcing to simulate the impact of the weakening of the THC. Results show a drop intemperature from North Europe, the northern middle East Asia, and northern East Asia and a significantreduction of precipitation in East Africa, the Middle East, the Indian Peninsula, and the Yellow RiverValley. This seems to support the idea that coldness and aridification at ca. 4.0 ka BP was caused by theweakening of the THC.     

大西洋热盐环流与气候突变

未来是否会发生由大西洋热盐环流崩溃引起的气候突变是目前极富争议性的全球气候变化问题。首先回顾了过去发生的气候突变事件以及若干成因假说,介绍了McGill地球系统模拟组的有关研究成果,并探讨了过去气候突变事件中的南北半球遥相关,最后讨论了未来是否会发生大西洋热盐环流崩溃引起的气候突变。对过去发生的大西洋热盐环流突变的研究表明,它产生的气候背景与现代气候以及将来进一步变暖的气候截然不同,因此不能凭过去发生过的突变事件来推论将来会发生类似的气候突变。尽管有的气候模式预测全球变暖会导致大西洋热盐环流减弱甚至崩溃而引发气候突变,但是目前全球气候系统的观测有许多不足之处,并且现代气候模式也存在较大的不确定性,因此尚不能确信现在的全球变暖会导致这样的气候突变。     

从地球系统的观点看气候突变

在地球的气候史中,经历了许多次气候突变事件。这些突变对地球表层系统的演变与进化产生了巨大影响。随着全球气候变暖的持续发展,人类赖以生存的气候环境正在经历重大变化。如何科学地理解和认识这些变化,尤其是气候突变。是人类寻求对策,应对气候变化,保护自己持续发展的科学基础。文中从地球系统的角度对气候突变的科学问题进行了探讨,认为对气候突变的研究不应仅局限于大气圈,而应该对地球系统的整体变化进行研究。     

近42年日照市气候变化分析

根据日照市1961-2002年的年降水量、年平均气温以及季降水量、季平均气温资料，利用Mann-Kendall突变检验法和滑动t检验法，分析了日照市近42年的气候变化特征。结果表明：年平均气温、年降水量均存在突变，其中，冬季气温突变最为明显，秋季略有突变但不明显，春、夏季气温没有突变现象；夏季降水量突变最为明显，秋季不明显，冬、春季没有突变现象。     

小兴安岭五营林区近49a气候变化趋势及突变分析

利用1958～2006年五营林区的地面资料,采用气候倾向率、累积距平、信噪比等气候统计方法,研究了近49a五营气候的变化趋势、气候突变和异常年份等。结果表明:五营林区近49a气候明显变暖,雪季升温较雨季快。降水量异常的年份增加,年降水量呈显著减少的趋势,-22.44mm/10a。前后10a降水中心生了明显的时空变化。雨季降水日数、年平均气温分别在1974年、1987年发生了气候突变。     

南海夏季风爆发前后低纬大气环流突变特征

利用1982～1996年15年平均的NCEP再分析资料,研究南海夏季风爆发前后低纬大气环流的突变特征。结果表明,东南亚地区对流层中上层厚度（温度）场、高低风场和大气层顶净辐射加热率（QRT）都有突变发生。海温场的变化相对其他要素较为缓慢,但也存在明显的转折点。QRT突变最早,其次是海温场变化出现明显转折,再是厚度（温度）场、低层风场突变,高层风场的突变最迟;低层风场突变最快,其次是厚度（温度）场,最后是QRT和高层风场。南海地区的降水,水汽场的突变发生在南海夏季风爆发前,而且突变较快。     

冬季NPO突变前后大气环流和我国天气气候的变化

用Mann-Kendall突变检验方法对北太平洋涛动10a际气候突变进行的研究指出，在1976年左右存在着一次明显的气候突变，而突变前后大气环流场及遥相关型均发生了显著的变化。进而研究了北太平洋涛动突变前后我国冬季天气气候的变化特征。结果表明，突变前(后)，我国冬季气温一致性偏低(高)，且北方地区的变化明显大于南方；而降水则呈现出长江中下游地区偏多(少)和华南偏少(多)的反相特征。此外，冬季NPO突变前，北太平洋涛动以3a和6～7a的周期为主；突变后，3a周期仍然存在，但5a和10a左右的周期开始占主导地位。     

2007年南海夏季风爆发前后大气环流的非典型性突变特征

南海夏季风爆发的一般特征是南亚高压移至中南半岛北部;西太平洋副热带高压连续向东撤出南海地区,移到120°E以东的热带洋面上;高（低）空东北（西南）气流占据南海大部分地区,相应的105°E附近的越赤道气流建立,南海季风槽形成并同时伴有对流降水的发展和温、湿等要素的突变。国家气候中心的监测表明,2007年南海夏季风于5月第5候爆发。该年季风爆发后,虽然源自热带地区的低空西南气流迅速占据南海上空,高空盛行东北气流,且南亚高压西移至中南半岛上空,但对流、高度场以及降水场的突变特征均很不明显,表现为季风爆发后南海上空的对流依然偏弱,副高没有马上撤离南海,同时华南地区的降水量也没有迅速增强。因此,2007年南海夏季风爆发前后大气环流的变化特征具有非典型性。     

初夏至盛夏东亚副热带西风急流突变早晚与东亚环流异常的关系

利用1961-2004年NCEP／NCAR再分析逐候资料和全国160站月平均降水资料,分析了初夏至盛夏东亚副热带急流北跳和急流中心西移发生早晚对7月东亚大气环流和我国降水的影响。结果表明,急流北跳时间与7月长江中下游地区降水异常正相关,急流中心西移时间则与7月淮河流域降水异常正相关,与华北和河套地区降水异常负相关。急流北跳时间与南亚高压和西太平洋副热带高压南北位置异常及高纬贝加尔湖以东高压脊强度相关;而急流中心西移时间与南亚高压和西太平洋副热带高压的东西伸展及贝加尔湖以西高压脊强度相关,在急流中心西移偏晚年,南亚高压西缩,贝加尔湖西南侧高压脊增强,南下至华北和河套地区冷空气偏强,且西太平洋副热带高压东撤,冷暖空气在淮河流域交汇,使得华北和河套地区降水减少而淮河流域降水偏多;偏早年情况与偏晚年情况相反。     

Enhancement of Winter Arctic Warming by the Siberian High over the Past Decade

In recent decades the Arctic surface air temperature(SAT) in autumn has been increasing steadily. In winter, however, instead of a linear trend, the Arctic SAT shows an abrupt change that occurred in 2004. During the years from 1979 to 2003, the first principle component(PC1) of winter Arctic SAT remains stable, and no significant increasing trend is detected. However, the PC1 changes abruptly from negative to positive phase in the winter of 2004. The enhanced Siberian high may have contributed to this abrupt change because the temporal evolution of Arctic temperature correlates significantly with sea level pressure variation in the northern Eurasian continent, and the atmospheric circulation anomaly related to the Siberian high from 2004 to 2013 favors a warmer Arctic. With the help of the meridional wind anomaly around the Siberian high, warmer air is transported to the high latitudes and therefore increases the Arctic temperature.     

MM5 Simulations of the China Regional Climate During the LGM.II: In uence of Change of Land Area, Vegetation, and Large-scale Circulation Background

Using a regional climate model MM5 nested to an atmospheric global climate model CCM3, a series of simulations and sensitivity experiments have been performed to investigate the relative LGM climate response to changes of land-sea distribution, vegetation, and large-scale circulation background over China.Model results show that compared with the present climate, the fluctuations of sea-land distribution in eastern Asia during the LGM result in the temperature decrease in winter and increase in summer. It has significant impact on the temperature and precipitation in the east coastal region of China. The impact on precipitation in the east coastal region of China is the most significant one, with 25%-50% decrease in the total precipitation change during the LGM. On the other hand, the changes in sea-land distribution have less influence on the climate of inland and western part of China. During the LGM, significant changes in vegetation result in temperature alternating with winter increase and summer decrease, but differences in the annual mean temperature are minor. During the LGM, the global climate, i.e., the large-scale circulation background has changed signi cantly. These changes have signi cant influences on temperature and precipitation over China. They result in considerable temperature decreases in this area, and direct the primary patterns and characteristics of temperature changes. Results display that, northeastern China has the greatest temperature decrease, and the temperature decrease in the Tibetan Plateau is larger than in the eastern part of China located at the same latitude. Moreover, the change of large-scale circulation background also controls the pattern of precipitation change. Results also show that, most of the changes in precipitation over western and northeastern parts of China are the consequences of changing large-scale circulation background, of which 50%-75% of precipitation changes over northern and eastern China are the results of changes in large-scale circulation background. Over China, the LGM climate responses to di erent mechanisms in order of strength from strong to weak are, the large-scale circulation pattern, sealand distribution, vegetation, CO2 concentration, and earth orbital parameters.     

Atmospheric and Terrestrial Water Balances of Labrador's Churchill River Basin,as Simulated by the North American Regional Climate Change Assessment Program

In an effort to understand the sources of uncertainty and the physical consistency of climate models from the North American Regional Climate Change Assessment Program (NARCCAP), an ensemble of general circulation models (GCMs) and regional climate models (RCMs) was used to explore climatological water balances for the Churchill River basin in Labrador, Canada. This study quantifies mean atmospheric and terrestrial water balance residuals, as well as their annual cycles. Mean annual atmospheric water balances had consistently higher residuals than the terrestrial water balances due, in part, to the influences of sampling of instantaneous variables and the interpolation of atmospheric data to published pressure levels. Atmospheric and terrestrial water balance residuals for each ensemble member were found to be consistent between base and future periods, implying that they are systemic and not climate dependent. With regard to the annual cycle, no pattern was found across time periods or ensemble members to indicate whether the monthly terrestrial or atmospheric root mean square residual was highest. Because of the interdependence of hydrological cycle components, the complexity of climate models and the variety of methods and processes used by different ensemble members, it was impossible to isolate all causes of the water balance residuals. That being said, the residuals created by interpolating a model's native vertical resolution onto NARCCAP's published pressure levels and the subsequent vertical interpolation were quantified and several other sources were explored. In general, residuals were found to be predominantly functions of the RCM choice (as opposed to the GCM choice) and their respective modelling processes, parameterization schemes, and post-processing.